Electrowinning plate.
This is almost certainly a plate that came from the electrolytic extraction of cobalt from its ore, or from an electrolytic purification process. Cobalt was dissolved in a solution and electricity was used to plate it out onto an electrode. Very pretty, very heavy.Source:eBay seller akilawolfContributor:Theodore GrayAcquired:5 June, 2003Price: $15.50Size: 1.2"Purity: 99.9%